An engine speed immediately after the start of an engine increases so as to significantly exceed a target idle speed, and the engine speed converges to the target idle speed after having reached its peak. This is because the engine is started in the state where the inside of an intake pipe at the start of the engine is under atmospheric pressure, and further in the state where a throttle opening degree (and an ISC valve opening degree) are large and accordingly an opening area of an intake passage is sufficiently secured. The reason why the engine speed converges to the target idle speed after having reached its peak is that the throttle opening degree (or the ISC valve opening degree) is controlled by feedback control of the engine speed.
With regard to a control device for an engine, the following conventional technology has been disclosed for preventing an engine speed immediately after the start of the engine from significantly exceeding a target idle speed. That is, until a peak of the engine speed is detected after the start of the engine, a control parameter different from an intake flow rate, which relates to an engine output, for example, ignition timing is set to a side on which the engine output decreases more than a parameter value at the time of a normal idle. Specifically, in the case of ignition timing control, the ignition timing is set to the retard side. After the peak of the engine speed has been detected, the control parameter is returned to the parameter value at the time of the normal idle (for example, Patent Document 1).
In addition, with regard to an ignition timing control device for an engine, a change amount of an engine speed immediately after the start of the engine is calculated as an area per unit time, and an advance amount of ignition timing is controlled in accordance with a value of the area (for example, Patent Document 2).    Patent Document 1: JP Patent Publication (Kokai) No. 2005-194902 A    Patent Document 2: JP Patent Publication (Kokai) No. 2000-249032 A